1. Field of the Invention
The invention relates generally to the field of radiation detectors used with well logging instruments. More specifically, the invention relates to systems and methods for stabilizing the gain of photomultipliers used with such detectors so that energy of the detected radiation can be accurately determined.
2. Background Art
Radiation detectors are used in a number of different types of well logging instruments. Well logging instruments in general are sensors enclosed in various types of housings such that the housing and enclosed sensors can be moved along a wellbore drilled through subsurface Earth formations. A record with respect to time and/or depth is made of the measurements made by the various sensors, and such measurements are used to generate images or other representations of the spatial distribution of certain physical parameters of the subsurface formations.
Radiation detectors known in the art used with well logging instruments includes scintillation detectors. Scintillation detectors include a scintillation crystal made from an optically transparent material that is sensitive to one or more types of radiation. One such crystal, sensitive to gamma radiation, is made from thallium-doped sodium iodide. Other scintillation crystals are made from materials such as bismuth germanate, gadolinium silicate, or lutetium oxyorthosilicate. See, for example, U.S. Pat. No. 5,660,627 issued to Menente et al and assigned to the assignee of the present invention. The foregoing crystal materials generate a flash of light when exposed to gamma radiation. The amplitude of the light flash corresponds to the energy of the gamma ray photon entering the crystal. A photomultiplier is typically coupled to the crystal. A photomultiplier includes a photocathode that releases electrons when light is imparted to the cathode. A series of intervening electrodes, called dynodes, are disposed along an electron path between the cathode and an anode. Each successive dynode is held at a higher voltage than the previous dynode, and the anode is held at the highest voltage. Electrons released by the cathode are attracted to the successive dynodes, each time causing the successive dynodes to emit a plurality of electrons for each incoming electron. By the time the electron “cascade” reaches the anode there may be several orders of magnitude more electrons than were originally released by the photocathode in response to the incoming light flash. The result is that an electrical pulse develops across the anode, the magnitude of which corresponds to the amplitude of the incoming light pulse, and thus to the energy of the gamma photon that entered the crystal.
The photomultiplier is typically coupled to electronic circuitry that measures the amplitude of each pulse generated by the photomultiplier. The pulse amplitudes and numbers of pulses having each determined amplitude are used to make inferences about the characteristics of the formations being evaluated by the well logging instrument, based on the assumption that the pulse amplitudes correspond to known gamma photon energies. Typically, the pulse amplitude measuring circuitry assigns a “channel” to detected pulse amplitudes that fall with a predetermined range. For each detected pulse falling within a particular channel, a counter corresponding to the channel is incremented. Thus, a spectrum of detected radiation may be determined by determining numbers of counts in each channel counter. Evaluating the actual energy of such detected radiation events requires that the channels are calibrated with respect to detected radiation energy level.
It is known in the art to calibrate a photomultiplier by including an energy reference in the crystal. For example, a small amount of cesium-137 as a calibration source may be used because it generates monochromatic gamma rays having energy of 662 thousand electron volts (keV). During operation of the radiation detector, it is known in the art to adjust the voltage applied to the anode and dynodes of a photomultiplier such that the voltage pulses attributable to gamma photons emanating from the calibration source are maintained at a selected measured pulse amplitude. Such selected value is typically related to the “channel” assigned by the pulse amplitude measuring circuit to the detected voltage pulse. Circuitry in the well logging instrument determines the channel of the calibration source energy peak, and adjusts the voltage applied to the photomultiplier to maintain the determined peak in a selected channel or “window” of contiguous channels.